1. Field of the Invention
The present invention relates to an image sensor including a semiconductor light receiving element.
2. Description of the Related Art
In general, an image sensor is formed of a light receiving portion pixel array including semiconductor light receiving elements. A photodetector of a photodiode type having a PN junction is widely used for the semiconductor light receiving element of each pixel included in the light receiving portion pixel array. Light entering each pixel is absorbed into a semiconductor substrate to generate carriers, and then the generated carriers are recombined at a depletion layer portion of the photodiode, with the result that an output of the semiconductor light receiving element is obtained as a voltage or a current.
Requirements for acquisition of high definition image data drove the size reduction of the pixel, making it difficult to obtain desired characteristics. In particular, degradation of characteristics due to crosstalk between pixels is conspicuous. Carriers generated immediately below a certain pixel diffuse in a lateral direction to be collected by a depletion layer portion of an adjacent pixel. The carriers are then recombined to become an unnecessary signal, resulting in an unstable output signal. In normal pixels, crosstalk components from adjacent regions are canceled with each other, and hence an output signal is less likely to be lowered. Particularly in a pixel that is arranged outermost, however, no cross component comes from an adjacent region, resulting in lowering of the output signal.
One countermeasure is to enhance the depth of the depletion layer of the photodetector so as to increase carrier collection efficiency. Another proposed method is to form a reflective insulating layer below a depletion layer so as to increase efficiency of collecting carriers into the depletion layer, to thereby suppress crosstalk (see, for example, Japanese Patent Application Laid-open No. Hei 5-206495).
The light with a long wavelength (red to infrared light), however, penetrates deeply into a Si substrate since the depth of the depletion layer can be increased to a limited extent (about 1.2=|m), disabling the suppression of the crosstalk between pixels due to lateral diffusion to an adjacent pixel. In the pixel configuration in which the reflective insulating film is formed below the depletion layer, incident light cannot be completely insulated, resulting in a problem of crosstalk due to carriers generated by transmitted light.